Polyethylene composition for blow molding having an improved swell behavior

Abstract

A polyethylene composition particularly suited for producing blow-molded hollow articles, having the following features: 1) density from 0.948 to 0.955 g/cm 3 ; 2) ratio MIF/MIE from 60 to 120; 3) MIF from 15 to 45 g/10 min.; 4) ER values from 2 to 5.0; said composition being obtainable by contacting a precursor polyethylene composition with a radical initiator.

What is claimed is: 
     
         1 . A polyethylene composition having the following features:
 1) ratio MIF/MIE from 60 to 120, preferably from 62 to 115, where MIF is the melt flow index at 190° C. with a load of 21.60 kg, and MIE is the melt flow index at 190° C. with a load of 2.16 kg, both determined according to ISO 1133-2:2011;   2) MIF from 15 to 45 g/10 min., preferably from 20 to 40 g/10 min.;   3) density from 0.948 to 0.955 g/cm 3 , in particular from 0.950 to 0.954 g/cm 3 , determined according to ISO 1183-1:2012 at 23° C.;   4) ER values from 2 to 5.0, preferably from 2.5 to 4.8, more preferably from 3 to 4.8;   ER is calculated from:
     ER =(1.781*10 −3 )* G′   
   at a value of G″=5,000 dyn/cm 2 ;   wherein:   G′=storage-modulus;   G″=loss-modulus;   both G′ and G″ being measured with dynamic oscillatory shear in a plate-plate rotational rheometer at a temperature of 190° C.;   said polyethylene composition being obtainable by contacting with a radical initiator a precursor polyethylene composition (I) having a ratio MIF/MIE 1 I ) lower than MIF/MIE 1) and preferably equal to or lower than 95, in particular from 95 to 50 or from 95 to 59; wherein the ratio 1)/1 I ), which is between the MIF/MIE 1) of the final polyethylene composition (after contacting with the radical initiator) and the MIF/MIE 1 I ) of the precursor polyethylene composition (I), is equal to or greater than 1.05, in particular from 1.05 to 1.8, or from 1.05 to 1.4.   
     
     
         2 . The polyethylene composition of  claim 1 , wherein the precursor polyethylene composition (I) has the following additional features:
 2 I ) MIF from 20 to 50 g/10 min., preferably from 25 to 45 g/10 min.;   3 I ) density from 0.948 to 0.955 g/cm 3 , in particular from 0.950 to 0.954 g/cm 3 ;   4 I ) ER values from 2 to 4, preferably from 2 to 3.5.   
     
     
         3 . The polyethylene composition of  claim 1 , consisting of or comprising one or more ethylene copolymers. 
     
     
         4 . The polyethylene composition of  claim 1 , wherein the precursor polyethylene composition (I) comprises:
 A) 30-70% by weight, preferably 40-60% by weight of an ethylene homopolymer or copolymer (the homopolymer being preferred) with density equal to or greater than 0.960 g/cm 3  and melt flow index MIE at 190° C. with a load of 2.16 kg, according to ISO 1133, of 8 g/10 min. or higher, preferably of 10 g/10 min. or higher;   B) 30-70% by weight, preferably 40-60% by weight of an ethylene copolymer having a MIE value lower than the MIE value of A), preferably lower than 0.5 g/10 min.   
     
     
         5 . The polyethylene composition of  claim 1 , obtainable by using, in the polymerization stage, a Ziegler-Natta polymerization catalyst. 
     
     
         6 . The polyethylene composition of  claim 5 , wherein the Ziegler-Natta polymerization catalyst comprises the product of reaction of:
 A) a solid catalyst component comprising Ti, Mg, chlorine and one internal electron donor ED selected from esters of aliphatic monocarboxylic acids (EAA) and another internal donor ED I  selected from cyclic ethers (CE) in an amount such that the EAA/CE molar ratio ranges from 0.02 to less than 20;   B) an organo-Al compound; and optionally   C) an external electron donor compound.   
     
     
         7 . The polyethylene composition of  claim 1 , having at least one of the following additional features:
 swell ratio equal to or higher than 140%, in particular from 140% to 190%;   FNCT 6 MPa/50° C., equal to or higher than 25h, in particular from 25 to 100h;   comonomer content equal to or less than 1.5% by weight, in particular from 0.1 to 1.5% by weight, with respect to the total weight of the composition;   MIP of 0.3 g/10 min. or higher, more preferably of 0.5 g/10 min. or higher, in particular from 0.3 to 4 g/10 min., or from 0.5 to 4 g/10 min., where MIP is the melt flow index at 190° C. with a load of 5 kg, determined according to ISO 1133-1 2012-03;   ratio (MIF/MIE)/ER, which is between MIF/MIE and ER, of equal to or less than 28, in particular from 28 to 15, or from 28 to 18;   long chain branching index LCBI equal to or greater than 0.55, more preferably equal to or greater than 0.58, the upper limit being preferably of 0.95 in all cases;   where LCBI is the ratio of the measured mean-square radius of gyration Rg, measured by GPC-MALLS, to the mean-square radius of gyration for a linear PE having the same molecular weight at a mol. weight of 1,000,000 g/mol;   η 0.02  equal to or greater than 30,000 Pa·s, more preferably equal to or greater than 32,000 Pa·s, in particular from 30,000 or 32,000 to 60,000 Pa·s;   ratio (η 0.02 /1000)/LCBI, which is between η 0.02  divided by 1000 and LCBI, from 50 to 70, more preferably from 52 to 70;   HMWcopo index from 0.5 to 5, in particular from 0.5 to 3.5;   
       where the HMWcopo index is determined according to the following formula:
   HMWcopo=(η 0.02   ×t   maxDSC )/(10{circumflex over ( )}5)
 
 
       where t maxDSC  is the time, in minutes, required to reach the maximum value of heat flow (in mW) of crystallization (time at which the maximum crystallization rate is achieved, equivalent to the t½ crystallization half-time) at a temperature of 124° C. under quiescent conditions, measured in isothermal mode in a differential scanning calorimetry apparatus, DSC. 
     
     
         8 . A process for preparing a polyethylene composition having the following features:
 1) ratio MIF/MIE from 60 to 120, preferably from 62 to 115, where MIF is the melt flow index at 190° C. with a load of 21.60 kg, and MIE is the melt flow index at 190° C. with a load of 2.16 kg, both determined according to ISO 1133-2:2011;   2) MIF from 15 to 45 g/10 min., preferably from 20 to 40 g/10 min.;   3) density from 0.948 to 0.955 g/cm 3 , in particular from 0.950 to 0.954 g/cm 3 , determined according to ISO 1183-1:2012 at 23° C.;   4) ER values from 2 to 5.0, preferably from 2.5 to 4.8, more preferably from 3 to 4.8;   said process comprising contacting with a radical initiator a precursor polyethylene composition (I) having a ratio MIF/MIE 1 I ) lower than MIF/MIE 1) and preferably equal to or lower than 95, in particular from 95 to 50 or from 95 to 59; wherein the ratio 1)/1 I ), which is between the MIF/MIE 1) of the final polyethylene composition (after contacting with the radical initiator) and the MIF/MIE 1 I ) of the precursor polyethylene composition (I), is equal to or greater than 1.05, in particular from 1.05 to 1.8, or from 1.05 to 1.4.   
     
     
         9 . The process of  claim 8 , wherein the precursor polyethylene composition (I) has the following additional features:
 2 I ) MIF from 20 to 50 g/10 min., preferably from 25 to 45 g/10 min.;   3 I ) density from 0.948 to 0.955 g/cm 3 , in particular from 0.950 to 0.954 g/cm 3 ;   4 I ) ER values from 2 to 4, preferably from 2 to 3.5.   
     
     
         10 . The process of  claim 8 , carried out by contacting the radical initiator and the precursor polyethylene composition (I) in an extruder device at a temperature of from 180 to 350° C. 
     
     
         11 . The process of  claim 8 , wherein the radical initiator is selected from organic peroxides. 
     
     
         12 . The process of  claim 11 , wherein the amount of added organic peroxide or peroxides corresponds to a content of initiator in the precursor polyethylene composition (I) of from 5 to 100 ppm by weight, in particular from 5 to 80 ppm by weight or from 5 to 50 ppm by weight. 
     
     
         13 . The process of  claim 11 , wherein the organic peroxide or peroxides are selected from 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexoxonane, or a representative of 3,6,9-trimethyl-3,6,9-tris(alkyl)-1,2,4,5,7,8-hexoxonanes in which the alkyl radical is propyl or ethyl. 
     
     
         14 . Manufactured article comprising the polyethylene composition of  claim 1 . 
     
     
         15 . The manufactured article of  claim 14 , in form of blow molded articles, in particular handleware.